Acidic rain is one of the recent factors which deteriorate the top-coating composition for automobiles. Coating films formed from acrylic or polyester resin-based coating compositions which have been most extensively used as top-coating compositions to date occur appearance defects, such as spotting, discoloration, etc., due to acidic rain and there are cases where these defects lead to film destruction.
On the other hand, coating compositions which give coating films with good resistance to acidic rain, such as silicone resin-based, fluororesin-based, or polyurethane resin-based coating compositions, are disadvantageous in that they are costly and poor in coating workability.
JP-A-61-143472 discloses a coating composition comprising a thermosetting acrylic resin having a Tg of 0.degree. C. or less, a hydroxyl value of from 20 to 130, and an acid value of from 6 to 40 and a butylated melamine resin as a crosslinking agent. (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). In this reference, there is a description that a small proportion of an isocyanate may be used in combination with the butylated melamine resin, but there is no description concerning effects obtained by the combined use of the two ingredients.
JP-A-60-67515 discloses a coating composition which comprises an acrylic copolymer containing .epsilon.-caprolactone-modified vinyl monomer and having a hydroxyl value of from 20 to 200 and a polyester resin, in which the polymers are heat-cured with a crosslinking agent which is an aminoaldehyde resin and/or an isocyanate. However, not only this reference fails to describe effects of the combined use of a melamine resin and an isocyanate, but also according this technique, an acrylic resin should be used in combination with a polyester resin.
JP-A-59-36112 discloses a coating composition which comprises an acrylic copolymer having a Tg of 50.degree. C. or less and in which in the case where this copolymer contains an active hydroxyl group or active amide group, a crosslinking agent that is a polyfunctional isocyanate, an aminoadehyde resin, or a mixture thereof may be used. However, this reference does not describe effects obtained by the combined use of these crosslinking agents.
The acidic resistance of coating films obtained by any of the above-described prior art techniques is insufficient and is lower than the level attained by the present invention.